IR-laser machining systems are widely used in industrial laser machining operations, such as cutting sheet-metal. The term IR-laser here refers to lasers delivering radiation having a wavelength of about 3 micrometers (μm) or greater.
A particularly preferred IR-laser for such cutting operations is a carbon-dioxide (CO2) laser delivering laser-radiation having a wavelength of about 10 μm. Such lasers are commercially available with output-power up to one kilowatt (kW) or greater.
One problem that can be encountered in CO2 laser sheet-metal cutting operations is instability of the laser output caused by feedback into the laser of laser-radiation reflected from an object being cut. This is particularly problematical in sheet-metal cutting, as most metals have a reflectivity greater than about 80% at wavelengths around 10 μm, even from surfaces that are not highly polished.
A usual solution for this problem is to provide an optical isolator in a beam-path between the laser and a workpiece being cut. An optical isolator is a polarization-sensitive device arranged to transmit radiation along a beam-path toward the workpiece being cut and reflect radiation returning along the beam-path.
Optical isolators for shorter-wavelength radiation, for example radiation having a wavelength of about 1 μm, are relatively simple devices, typically employing a magneto-optic birefringent crystal cooperative with a polarizer and an polarization-analyzer. An alternative arrangement is required for wavelengths greater than about 3 μm, as there are no practical magneto-optical materials (Faraday rotators) that will efficiently transmit these wavelengths.
Optical isolators for these long wavelengths are made from a series of multilayer thin-film reflectors which are polarization-dependent for reflected amplitude or reflective phase. These reflectors are all used with radiation incident thereon at non-normal incidence. Those with polarization-dependent reflective phase provide the function of the Faraday rotators in shorter wavelength isolators.
The reflectors must be coated with very precise tolerances, as there is effectively no means for adjusting the reflected phases at a fixed angle of incidence. Such an isolator can add significantly to the cost of an IR laser-machining system. This is because of the number of components required, the cost of meeting required reflectivity and phase tolerances for the components, and the precision required for aligning the components. There is a need for reducing the cost of optical isolation in IR laser-machining systems.